Advice regarding isolator for 3ph TT system

[Raptor0014]

Morning all,

I’m scratching my head with this and granted I don’t do a lot of commercial work. Hence I thought I’d ask.

Setup is a pub with a 3ph supply and it’s TT

So it goes

Incoming head - 3 x 100a cutout

Meter

3 pole isolator for L1, L2 & L3

L1 to single phase into plastic DB

L2 to single phase into plastic DB

L3 to single phase into plastic DB

Neutral into a Henley then to each single phase DB

However my understanding is that with a TT system it must be a 4 pole (TP&N) isolator? Only TNS & TNCS don’t need to have the neutral isolated.

Is there any reason why a TT system would be able to have an exception? I’ve no previous paperwork to look at to see a reason why.

Advice appreciated before I go back to them to tell them they need a 4 pole isolator.

 

[westward10]

For a TN system the neutral is deemed at earth potential so there is no need to isolate it but this is not the case for TT so the isolator must also switch the neutral.

 

[Raptor0014]

For a TN system the neutral is deemed at earth potential so there is no need to isolate it but this is not the case for TT so the isolator must also switch the neutral.

That was my understanding. But I can’t understand how this has been signed off and had previous EICRs if it is not done right.

Hence I was wondering if I was missing something.

 

[westward10]

Have you seen previous EICRs it may well be on them.

 

[Raptor0014]

Have you seen previous EICRs it may well be on them.

I haven’t as they don’t have one.

But I would have thought, knowing the pub owner, that they would have acted on it if it was listed.

 

[westward10]

Without seeing it there is no way of knowing.

 

[Raptor0014]

But to go back to my question.

There is no reason why a TT wouldn’t need the neutral to be disconnected by an isolator?

 

[westward10]

But to go back to my question.

There is no reason why a TT wouldn’t need the neutral to be disconnected by an isolator?

No it should be isolated.

 

[Raptor0014]

Just had a good chat with NAPIT technical and they agree it should be neutral disconnect being a TT.

So I’ll go back and tell the isolator needs swapping out.

 

[loz2754]

Indeed. And that's without mentioning the requirement for all pole RCD protection.

 

[Raptor0014]

Indeed. And that's without mentioning the requirement for all pole RCD protection.

Intention is to fit a 100A 100ma Time delayed Type A TP&N isolator.

Picked up this MCG one today.

 

[westward10]

All insulated DBs I wouldn't be putting in a main switch rcd just fit a four pole DNO style switch.

 

[Raptor0014]

All insulated DBs I wouldn't be putting in a main switch rcd just fit a four pole DNO style switch.

I initially was going to do that but one of the DB's needs to be replaced so will be a metal one.